The present invention relates to shrink articles and items and a method of making heat recoverable i.e. shrinking, shrunk and shrinkable objects.
Shrink or heat recoverable articles and objects of the kind to which the invention pertains are usually manufactured in that a thermoplastic synthetic material is first shaped in accordance with the (ultimately) desired shape; next, the thermoplastic synthetic material is chemically or physically cross linked whereupon the object thus attained is heated above the crystalline melting point. The hot object is then stretched in the desired direction, and while maintaining the stretched expanded condition, it is cooled. The cooling "freezes" the stretched and extended state as far as the physical configuration is concerned. However as the article is being reheated it returns to i.e. recovers the original dimensions the object had prior to the stretching and extending process.
Articles, items and objects of the kind described above are used for a variety of purposes. One purpose is the covering of joints in electrical cable or tube configuration. In other words the shrink article is used as a covering sleeve for splicing or connecting areas and zones in cable and tubular configurations. It is a drawback here that the strength particularly the tear strength, of such a sleeve is indeed very low when in the extended and stretched state at elevated temperature as well as for the nearly as high recovery temperature, even though "hot plus extended" is just temporary state; still, if there is for some reason a pointed area or a sharp edge or the like and if the article engages such a point edge or the like, there is a great danger of tearing and cutting.
In order to increase the tear strength of these kinds of articles it is known to embed threads, fibers or the like of high strength material in the wall of the shrink article. For example, European patent 115905 (corresponding US case KEG/L982, Ser. No. 379,093, filed Jul. 13, 1989) now U.S. Pat. No. 5,141,812 discloses a heat recoverable fabric made of heat recoverable fibers running on one direction, and transverse thereto and interwoven glass fibers. This fabric, textile, mesh or webbing is embedded in a matrix of synthetic. The danger of tearing is indeed quite reduced in this case but it is a disadvantage that on recovery only the heat recoverable fibers will in fact shrink and will carry the matrix along. The glass fibers on the other hand are effective only transverse to that direction of shrinking.
Another solution to the aforementioned problem is found in an application by me and others, Ser. No. 379,093, filed Jul. 13, 1989 now U.S. Pat. No. 5,141,812. Here is disclosed a fabric or mesh or other woven etc. configuration, made of strands and which are embedded in a synthetic matrix. Each strand is stretchable and is comprised of a central, synthetic, heat recoverable core, surrounded by a strong thread of helical configuration, possibly in a mesh or other pattern spun or threaded around the core. This helical "layer" is made of high strength thread such as glass i.e. it is not heat recoverable.
As the core is stretched the pitch of the helical (e.g. glass fiber) "threading" is increased while simultaneously the inner diameter of the helical configuration is reduced, but so is the outer diameter of the stretched core. Thus, owing to the helical configuration of the initial reenforcing threading one can speak here of an extension reserve which may amount to 400% or even more. In the extreme case of course the helical glass fiber is stretched to almost straight configuration.
The fabric using such strands may be comprised of a plurality of parallelly extending strands which are of the kind described and which are interwoven or pleated with transversely extending glass fibers. On shrinking only the matrix pulls and the cores of the strands, and they shrink in their direction of extension while the glass fibers on the cores and those interwoven transversely to them, are in fact reinforcing the configuration respectively in the direction of shrinking as well as transversely thereto.